platform/x86: asus-wmi: add a helper for device presence

Factor out the WLAN LED and lightbar LED presence checks into a
helper function, which will also be used by the upcoming CPU fan device
support.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>